The present invention relates to oligosaccharide synthesis, and particularly to the enzyme-catalyzed transfer of a mannosyl group to an acceptor molecule coupled with regeneration of GDP-mannose (GDP-Man).
The designed enzymatic synthesis of oligosaccharide-containing molecules has recently gained prominence in the art as greater numbers of glycosyltransferase enzymes have become available to skilled workers. See, for instance, U.S. Pat. Nos. 5,180,674 and 5,278,299.
Indeed, U.S. Pat. No. 4,569,909 to Seno et al. teaches the use of uridine diphosphate-N-acetylglucosamine 4-epimerase to epimerize UDP-GlcNAc into an equilibrium mixture of UDP-GlcNAc and UDP-GalNAc. That mixture, after boiling to stop enzymic activity and centrifugation to remove the denatured enzyme, provided a xe2x80x9croughxe2x80x9d preparation of UDP-GalNAc that was used with an xcex1-N-acetylgalactosaminyl transferase referred to as xe2x80x9cA-transferasexe2x80x9d to convert Type O red blood cells into Type A red blood cells.
Seno et al. began each of their reactions with UDP-GlcNAc, a compound that is relatively difficult to prepare and store in large quantity. Seno et al. also had no concept of a regeneration step in which UDP-GalNAc or any other sugar-linked nucleotide is recycled.
Oligosaccharide synthesis based on sugar nucleotide-dependent glycosyltransferases proceeds regio- and stereoselectively under mild reaction conditions without multiple protection and deprotection step. For review in the field, see Toone, et al., Tetrahedron, 45:5365 (1989); David et al., Adv. Carbohydr. Chem. Biochem., 49:175 (1991); Drueckhammer et al., Synthesis, 7:499 (1991) and Ichikawa et al., Anal. Biochem., 202:215 (1992). Glycosyltransferases, however, are difficult to obtain (xcex2-1,4-galactosyltransferase is the only one commercially readily available), and the enzymatic synthesis requires sugar nucleotide regeneration for large-scale processes. Wong et al., J. Org. Chem., 47:5416 (1982); Ichikawa et al., J. Am. Chem. Soc., 113:4698 (1991); Ichikawa et al., J. Am. Chem. Soc., 113:6300 (1991); Wong et al., J. Org. Chem., 57:4343 (1992); Ichikawa et al., J. Am. Chem. Soc., 114:9283 (1992).
More than 50 glycosyltransferase genes have been cloned and sequenced from bacteria, yeast and mammalian cells, and documented in the Genebank (IntelliGenetics, Inc.). The availability of these sequences provides researches an opportunity to overexpress glycosyltransferases in large quantities and use them for oligosaccharide synthesis. Of the eight sugar nucleotides commonly used as donor substrates for mammalian glycosyltransferases, five; i.e. UDP-Glc, UDP-Gal, GDP-Fuc, CMP-NeuAc and UDP-Glucuronic acid, have the regeneration system available for large-scale processes. For a review, see Wong et al., Pure and Appl. Chem., 64:1197-1202 (1992).
The enzymes required for the regeneration of GDP-Man, UDP-GlcNAc and UDP-GalNAc have been reported, [Heidlas et al., Acc. Chem. Res., 25:307-314 (1992); Wong et al., Pure and Appl. Chem., 64:1197-1202 (1992)] although regeneration of these sugar nucleotides has not been demonstrated. As part of efforts to develop glycosyltransferase-based enzymatic procedures for the synthesis of complex oligosaccharides and glycopeptides are disclosed hereinafter, as are the overproduction and specificity study of the soluble catalytic domain of an xcex11,2-mannosyltransferase (ManT), [Lewis et al., Glycobiology, 2:77 (1992)]. The application of this enzyme coupled with regeneration of guanosine 5xe2x80x2-diphosphomannose (GDP-Man) to the synthesis of mannose-containing oligosaccharides and glycopeptides are also disclosed hereinafter, and by the present inventor and colleagues in Wong et al., Pure and Appl. Chem., 65(4):803-809 (1993) and Wang et al., J. Org. Chem., 58:3985-3990 (1993).
The present invention contemplates an enzymatic process for transferring a mannosyl group to an acceptor. Transfer of any glycosyl group is referred to as glycosylation, and transfer of a mannosyl group is mannosylation. The contemplated process also recycles the GDP that is formed upon glycosyl transfer to form further amounts of GTP and thereby regenerate the GDP-Man.
In accordance with a contemplated process, an aqueous reaction medium is formed by admixing the following ingredients in an aqueous medium in a single vessel:
(i) a mannosyl (mannose) 1-phosphate (Man-1-P);
(ii) GDP-Man pyrophosphorylase that catalyzes the formation of GDP-Man from Man-1-P in the presence of GTP;
(iii) a mannosyltransferase;
(iv) an acceptor for the mannosyl-transferase of (iii); and
(v) a guanosine diphosphate (GDP) recycling system that includes (a) GDP, GTP or both, (b) a phosphate donor, and (c) a kinase to transfer a phosphate group from the phosphate donor to GDP to form GTP. Each of the enzymes of (ii), (iii) and (v) is present in a catalytic amount. The aqueous reaction medium so formed is maintained at a pH value of about 5 to about 10 at a temperature of about zero degrees C. to about 40xc2x0 C. for a time period sufficient for said acceptor to be glycosylated. The glycosylated acceptor that is formed is preferably recovered.
Also contemplated herein is a recombinant, water-dispersible xcex11,2-mannosyltransferase, its isolated DNA, an expression vector containing that isolated DNA such as pManflag20, and transformed E. coli that contain that vector.
Abbreviations
The various saccharides discussed herein are frequently discussed in their usually used abbreviations. Those abbreviations and saccharide names are listed below as monosaccharides.
GalNAc=N-acetylgalactosamine
Glc=glucose
GlcNAc=N-acetylglucosamine
Man=mannose
Man-1-P=mannose 1-phosphate
Man-6-P=mannose 6-phosphate